Sound

"G
IVE a light blow to a wine-glass. The glass will
ring, giving forth a sound, weaker or stronger, lower
or higher, according to the quality and size of the
glass. The sound lasts but a moment, and then ceases.
Strike the glass once more and while it is still
ringing touch the rim with your finger. Instantly all
is still again; not a sound from the glass. Why does it
ring when struck, and why does it stop ringing at the
touch of a finger? Before replying let us experiment
with other resonant objects.

"A violin-string twangs on being scraped by the bow or
plucked with the finger, and while it thus gives forth
its note it is seen to vibrate rapidly. So rapid,
indeed, is its vibration that it appears to fill the
entire space between its extreme positions, with the
result that it presents a swollen appearance in the
middle, after the manner of a spindle. With the
cessation of its vibration it becomes silent. It also
falls silent immediately at the touch of a finger.

"A bell rings on being struck by its clapper, and if
observed closely the substance of the bell will be seen
to tremble in an unmistakable manner. Place your hand
on the bronze and you will experience a disagreeable
sensation almost amounting to pain,
due to the vibration of the metal. Finally, if your
hand continues to rest on the bell, the vibration will
cease and with it the sound.

"Let us try something still more remarkable. Take a pin
by its pointed end and bring the head very close to a
ringing wine-glass or bell. You will hear a rapid
succession of little taps. Whence do they come? From
the glass or the bell-metal striking the pin a series
of quick blows as long as the ringing continues. They
come from the lively trembling of the sonorous object.

"It is unnecessary to cite other examples; these three
will suffice. They show that, in order to give forth
sound, a wine-glass or a bell or a
violin-string—in short, any object
whatever—must be made to tremble or vibrate with
great rapidity. The sound is heard as long as the
vibration continues, and ceases when the vibrating
object returns to a state of rest. That explains why
the wine-glass and the violin-string stop sounding at
the touch of a finger, and why the bell will not ring
if you rest your hand against it. The finger in the one
instance, the hand in the other, stop the resonant
trembling, and in so doing arrest the sound. Motion
causes the sound; rest brings silence.

"To this rapid motion back and forth is given the name
I have already used, vibration. An object from which
comes a sound is in vibration; it vibrates. Each of its
backward and forward movements, too rapid for the eye to
follow, is a vibration; and the quicker these
vibrations, the higher the note
sounded; the slower they are, the lower the note. In a
word, sound is motion and its place in the scale
measures the rapidity of that motion.

"In order to be heard, this sound, this motion, must
reach us. The hand detects it in its own way when,
resting on the vibrating bell, it experiences a very
disagreeable thrill; the finger becomes conscious of it
in a peculiar manner when, touching the violin-string,
it feels a ticking sensation. But how does the ear
contrive to receive the sound when it is at a distance
from the resonant object and apparently in no sort of
communication with it?

"At this point let me invite you to join me in a sport
very familiar to you all. We will take a big stone and
drop it into a calm sheet of water. Around the place
where the stone struck the water there is instantly
formed a circle, then another and another, and so on
indefinitely; and all these circles, described about
the same center and as regular as if drawn with a pair
of compasses, grow larger and larger, in successive
rings, until they die out at a long distance from their
common center, if the sheet of water is large enough.

"Now, do those circles really chase one another over
the water's surface as they appear to? One would
certainly think so. You remember how fast they go, one
ring after another, each in apparent haste to catch up
with its predecessor. But the pursued always keep clear
of the pursuers, and the distance between the rings
remains the same. And so the fact is they are not
really chasing one another;
they are not, in fact, moving at all; but they have
that deceptive appearance, and it will not be hard for
us to understand why.

"Let us drop a straw or a dry leaf upon the surface of
the water. When one of these concentric circles passes
like a wave, the straw or leaf is lifted up, after
which the wave goes on and leaves it, and it sinks down
again, remaining exactly where it was in the first
place. Thus it is proved that the water does not move
forward at all, for if it did it would carry with it
the straw or leaf on its surface.

"What, then, are those waves? Mere palpitations of the
water as, without changing its place, it gently rises
and falls, thus producing a succession of alternate
billows and furrows which appear to go chasing after
one another. Watch a field of wheat when the wind
blows. Its surface undulates in waves that seem to move
forward although the wheat-stalks remain firmly rooted
in the ground. Of like sort is the apparent movement of
a sheet of smooth water when disturbed by the fall of a
stone.

"The circles on the water and the undulations of the
wheat-field explain to us the nature of sound. Every
object emitting a sound is in rapid vibration, and each
of these vibrations causes a shock to the surrounding
air and produces a wave which spreads out in all
directions, immediately followed by a second, a third,
and countless others, all resulting from as many
successive vibrations.

"In the air thus shocked by the vibrating body there
takes place exactly what we see in the sheet of smooth
water disturbed by a falling stone and in the
wheat-field ruffled by the wind. Without changing its
place the air undergoes an undulatory movement which is
transmitted to great distances. In other words,
air-waves are formed which propagate themselves in
every direction at once through the atmosphere, thus
taking the form, not of circles, but rather of hollow
spheres, all having a common center.

"These air-waves are not visible to us, because air
itself is invisible; but they are none the less real,
just as real as waves of water and undulations of a
wheat-field. If the eye cannot see them, the ear can
hear them, for it is from them that sound comes. Hence
they are called sound-waves. The ear hears when
sound-waves reach it from any vibrating object."